1. Field of the Invention
The invention relates to a method of treating and/or preventing corrosion of steel, or other metal, in reinforced concrete structures. More particularly, the invention relates to a method of changing conditions on the surface of the steel from a condition of non-uniform surface potential where there is corrosion to a condition of substantially uniform surface potential where the corrosion is inhibited or prevented from forming, Specifically, the invention relates to a method of eliminating differences in surface potentials through applying continuously monitored, and uniquely adjusted, low voltage DC current to the steel reinforcement through an anode system which may, optionally, be temporarily installed on the surface of the concrete.
2. Summary of Related Art
A primary cause of the deterioration of reinforced concrete structures, such as bridge surfaces and substructures, is the corrosion of reinforcing steel due to exposure to chloride ions from salts used to de-ice the surface of the concrete or sea water. Chloride causes active corrosion by destroying the natural passivity of steel in the alkaline environment of concrete.
Concrete is a porous material. Steel placed in fresh concrete will not corrode because the naturally high pH level of the concrete causes a passivating oxide film to form on the surface of the steel, preventing it from corrosion. Minor imperfections from the concrete placement operation, as well as varying conditions during the hydration process, will result in variations in pore distribution in the concrete, as well as introduce voids. Voids and non-uniform pore distribution will result in variations in the permeability of concrete.
The variations in permeability, and with normal aging through exposure to the forces of nature, as indicated above, gradually change the environment within the concrete, from an environment where corrosion cannot take place to one where it can. Since concrete is not totally homogenous, and the concrete cover is not uniform, some areas will be affected before other areas of concrete. In these affected areas, the combination of the environmental exposure, ingress of chlorides and carbonation will cause a shift in the surface potential of the steel and the steel will experience a gradual loss of passivation that is protecting it against the onset of corrosion. Corrosion will now commence.
Since corrosion is an electrochemical process, the differences in the surface potential cause a low level electric current to pass between areas of different potentials, including surrounding concrete which acts as an electrolyte. Anodic and cathodic areas are established. In the anodic areas, ferrous ions leave the steel surface and react with available oxygen or hydroxides, forming corrosion products as ferrous oxides and/or hydroxides. Volume increases, resulting in the concrete cracking and spalling.
The prior art addresses the foregoing issues by using two systems: cathodic protection and chloride removal. Cathodic protection is designed to stop corrosion through passing a current equal to, or slightly stronger than the corrosion current, in the opposite direction. The chloride removal systems are directed to maintain a potential difference between an externally mounted anode and the reinforcing steel for the purpose of removing chloride ions through ion migration, The amount of electrical energy passed is designed to allow for a defined amount of chloride to be removed, and set levels defined as having no negative side effects.
U.S. Pat. No. 5,141,607 discloses a method and apparatus for treating reinforced steel for removing chlorides by placing an electrode adjacent an outer surface of the reinforced concrete structure, wetting the electrode and surface with an aqueous electrolyte and applying direct current between the reinforced steel and electrode to cause migration of the chloride ions.
U.S. Pat. No. 5,228,959 discloses a process for rehabilitating internally reinforced concrete by electrolytic ion migration by applying a removable adherent electrolyte to a surface area on the concrete and intermittently impressing and discontinuing a DC voltage between the internal reinforcement and electrode structure while measuring the potential difference of the internal reinforcement against a reference electrode structure.
U.S. Pat. No. 5,015,351 discloses a de-watering procedure for removing water from saturated, porous building materials, such as concrete wherein a controlled, cyclical voltage is applied to an electrode system for osmotic migration of water from an anode located inside the structure or in contact with it to a cathode, U.S. Pat. No. 5,049,412 discloses a method of re-alkalizing concrete in which carbonation has occurred by applying a water tight adherent coating to the surface of the concrete and saturating the coating with water to affect diffusion of alkaline materials from the areas of less carbonation to areas of more carbonation in the concrete. The concrete can then become re-alkalized.
The prior art does not address a method to stop corrosion by eliminating the differences in corrosion potentials, as is defined in the present invention. This method is most efficient in addressing the corrosion of reinforced steel in concrete.
It is an object of the present invention to solve the problems of the prior art enumerated above and below.